Method for Minimizing Delays While Drilling Using a Magnetic Ranging Apparatus

Abstract

A method and system is provided for minimizing delays in a magnetic ranging method. Delays are minimized by establishing synchronicity between triggering of a three-axis magnetometer and energisation of a solenoid assembly deployed in the borehole being drilled and the first borehole, respectively. Synchronicity enables measuring various components of the alternating magnetic field created by energisation of the solenoid assembly by the magnetometer, the moment the solenoid assembly is energised. The recorded components are used for computation of steering data for drilling the second borehole relative to the first borehole. The steering data can be determined at the surface or downhole.

We claim: 
     
         1 . A method for minimizing delays in a magnetic ranging method while drilling a second borehole relative to a previously-drilled first borehole, the second borehole having a three-axis magnetometer and a three-axis accelerometer deployed therein and the first borehole having two magnetic field sources, a first magnetic field source and a second magnetic field source, the first and second magnetic field sources being spaced by a rigid, non-magnetic spacer of known spacer distance, the method comprising:
 positioning the two magnetic field sources in the first borehole approximately opposite the magnetometer in the second borehole;   starting a survey with the magnetometer being rotationally stationary:
 triggering the magnetometer using electromagnetic telemetry (EM) synchronous with energizing of the two magnetic field sources from surface to produce an alternating magnetic field in the second borehole; 
 measuring components of the alternating magnetic field by the magnetometer as soon as the two magnetic field sources are energised; 
 measuring three-axis acceleration data from the three-axis accelerometer; 
   determining distance, direction and orientation of the second borehole relative to the first borehole using the measured components of the alternating magnetic field, the three-axis acceleration data and spacer; and   drilling the second borehole using the determined distance, direction and orientation.   
     
     
         2 . The method of  claim 1  wherein the simultaneous energisation of the first and second magnetic field sources and triggering of the three-axis magnetometer further comprises:
 synchronising triggering of the magnetometer with the energisation of the two magnetic field sources; the two magnetic field sources being activated by energizing the first magnetic field source and measuring components of a first magnetic flux density measured by the magnetometer, and then energizing the second magnetic field source and measuring components of a second magnetic flux density measured by the magnetometer. 
 
     
     
         3 . The method of  claim 1  wherein the triggering of the three-axis magnetometer is synchronised with the energisation of the first magnetic field source. 
     
     
         4 . The method of  claim 2  wherein determination of the distance, direction and the orientation using the measured components further comprises:
 combining the known spacer distance, the measured components of the first and second magnetic flux densities and the three-axis acceleration data using successive approximation inversion to produce the distance, direction, and the orientation of the second borehole relative to the first borehole. 
 
     
     
         5 . The method of  claim 1  wherein the two magnetic field sources are energised by a DC current and the energizing of and comprises alternating the first and second magnetic field sources in both positive and negative states. 
     
     
         6 . The method of  claim 1  wherein the energizing of the first and second magnetic field sources further comprises:
 alternating the first magnetic field source in both positive and negative states and measuring components of a first magnetic flux density; 
 alternating the second magnetic field source in both of the positive and the negative states and measuring components of a second magnetic flux density; and 
 combining the measured components of the first and second magnetic flux densities further comprises obtaining a difference between the first magnetic flux density in the positive and negative states and obtaining the difference between the second magnetic flux density in the positive and negative states. 
 
     
     
         7 . The method of  claim 1  further comprising:
 drilling the second borehole with a drilling assembly having the magnetometer incorporated therewith. 
 
     
     
         8 . The method of  claim 7  wherein the drilling assembly is supported on a coiled tubing and mud motor holding the magnetometer rotationally stationary while drilling. 
     
     
         9 . The method of  claim 1  further comprising conveying the two magnetic field sources along the first borehole using a tractor. 
     
     
         10 . The method of  claim 1  further comprising initiating energizing of the two magnetic field sources by wireline. 
     
     
         11 . The method of  claim 5  further comprising alternating the positive and negative states at frequencies at less than 1 Hz. 
     
     
         12 . The method of  claim 11  further comprising alternating the positive and negative states at frequencies at about 2 times per minute. 
     
     
         13 . The method of  claim 1  further comprising:
 providing magnetically permeable end caps on pole ends of each magnetic field source of the two magnetic field sources; 
 locating the two magnetic field sources in a cased first borehole; 
 producing alternating magnetic fields by energising the two magnetic field sources; and 
 conducting the alternating magnetic fields through the end caps for accurate location of the two magnetic field sources in the first borehole. 
 
     
     
         14 . The method of  claim 13  wherein an air gap between the two magnetic field sources and the cased first borehole is minimised by providing end pieces of a larger diameter than each of the two magnetic field sources on the ends thereof.